I. Field of the Invention
This invention relates generally to polysomnograhy (PSG) and more particularly to improved respiratory excursion sensing belts and an associated adapter that allows the improved sensing belts to be used with respiratory inductance plethysmography (RIP) based PSG machines. It bridges two incompatible technologies, piezoelectric (a high impedance capacitive output voltage due to stressing of a PVDF film) and low impedance inductor (which has its inductance changed when the RIP belt is stretched).
II. Description of the Prior Art
In a polysomnographic study, a variety of physiologic parameters must be measured. One of the most important assessments, breathing, is obtained by measurement of nasal and/or oral airflow in tandem with measurements of chest and abdominal wall movement.
An important task in securing and interpreting a polysomnogram is to assess whether apnea is present and to distinguish between obstructive and central apnea. Obstructive apnea is defined as an absence of airflow in the presence of continued effort to breathe. While this is a fairly straightforward definition, physiological assessment of obstructive apnea can be challenging. The essential task is to demonstrate effort to breathe in the absence of significant airflow.
Respiratory effort is directly measured by esophageal manometry. Esophageal pressure (Pes) is measured by having the patient swallow a pressure catheter which then resides in the esophagus throughout the sleep study. Rhythmic fluctuations in thoracic pressure in the absence of significant nasal and oral airflow are the best “proof” of the presence of obstructive apnea. In clinical practice, however, esophageal pressure is bothersome to most patients and is therefore not used routinely. A reasonable surrogate measure of respiratory effort can be obtained by measuring changes in chest and/or abdominal volume, also known as plethysmography. Changes in lung volume are most accurately measured using spirometry equipment, in which lung volumes and flow rates are determined by having the patient breathe through a pneumotachograph, and are unsuitable for polysomnography.
There are three primary methods of non-invasive chest and abdominal plethysmography in current use: measurement of changes in elastic belt tension, measurement of changes in transthoracic electrical impedance and measurement of changes in electrical inductance.
An elastic belt fastened around the chest or abdomen will exhibit a change in tension as the chest or abdomen expands or contracts. This change in tension can be easily measured and converted to a voltage by a variety of methods. The most common method in current use is a piezoelectric sensor, i.e., a crystal that directly generates a voltage when compressed or stretched. This method, while simple and inexpensive, is subject to trapping artifacts where a portion of an elastic belt becomes “trapped” as a person turns from one side to another during sleep, resulting in variable tension along the belt circumference. As a result, this method can both significantly under and/or overestimate the actual degree of chest or abdominal movement in addition to creating a false signal when the belt tension suddenly changes with a change in body position.
In the case of impedance plethysmography, the human body is a fairly poor conductor of electricity. It presents a fairly high impedance to electrical current flowing through it. This impedance changes as the cross-section of the body expands and contracts, allowing qualitative measurement of thoracic and abdominal movement during breathing. A plurality of electrodes are attached to the skin. A weak alternating current is passed through these electrodes, allowing the impedance to be measured. This method yields a non-linear signal, thus is useful only as a qualitative measurement of chest or abdominal movement. In addition, this signal is prone to movement artifact and cardiac artifact that can present challenges in discerning when the signal actually represents chest wall contraction or expansion due to breathing effort.
Respiratory inductance plethysmography (RIP) relies on the principle that a current applied through a loop of wire generates a magnetic field normal to the orientation of the loop (Faraday's Law) and that a change in the area enclosed by the loop creates an opposing current within the loop directly proportional to the change in the area (Lenz's Law). An elastic belt into which a zigzagging (coiled) wire is sewn is worn around the chest or abdomen. An alternating current is passed through the belt, generating a magnetic field. The frequency of the alternating current is set to be much greater than the typical respiratory rate in order to achieve adequate sampling of the respiratory effort waveform and in order to monitor the change in inductance due to breathing reliably. The act of breathing changes the cross-sectional area of the patient's body, and thus changes the shape of the magnetic field generated by the belt, inducing an opposing current that can be measured, most easily as a change in the frequency of the applied current. The signal produced is linear and is a fairly accurate representation of the change in cross-sectional area. In addition, RIP does not rely on belt tension, thus is not affected by belt trapping.
The American Academy of Sleep Medicine (AASM) has generally recommended the use of inductive sensing belts for measuring abdominal and thoracic circumferential changes due to respiration. These elastic belts incorporate an elongated, stretchable bent wire embedded therein that is driven by an oscillator whose frequency varies with inductance changes due to stretching and relaxing of the belts during breathing. It relies upon the self inductance and mutual inductance properties of the wires and their embedding in a stretchable belt.
Problems due to undetected wire breakage have resulted in aborted PSG procedures so frequently that the inductive belts are falling out of favor with many sleep lab professionals. An alternative respiratory belt is based on the piezoelectric properties of polyvinylidene fluoride (PVDF) films. An example of a respiratory belt based on PVDF technology is described in published U.S. patent application 2008/0275356, the contents of which are hereby incorporated by reference as if fully set forth herein.
While PVDF based respiratory belts produce a robust signal output linearly proportional to the elongation changes, they tend not to be directly compatible with all PSG machines that have been designed to accommodate RIP belts. This has inhibited the market acceptance of PVDF based respiratory belts.
It is accordingly an object of the present invention to provide an adapter circuit that will accept the piezo signal from a PVDF film transducer in a respiratory belt and produce an output that emulates that produced by RIP belts so that the PVDF belt can be used with existing PSG machines already ubiquitously present in the field.